1. Field of the Invention
The present invention relates to a transcutaneous blood gas sensor, which can continuously measure a gas concentration in the blood of the human body by attaching a sensor onto the skin.
2. Description of the Art
A well-known conventional transcutaneous blood gas sensor is disclosed in Japanese Utility Model Publication Gazette No. 31220/1984, for example. FIG. 11 is a sectional view showing an example of such a conventional transcutaneous blood gas sensor. The conventional gas sensor as shown in FIG. 11 has a sensor housing 1, a membrane holder 4 and a ring-shaped collar 6. The membrane holder 4 is located between the sensor housing 1 and the ring-shaped collar 6, which is threadedly connected to the sensor housing 1 to mount the membrane holder 4 on the sensor housing 1. The sensor housing 1 is provided in its periphery with a threaded part 8 and the ring-shaped collar 6 is provided in its inner periphery with a threaded part 7 for such threaded connection.
The sensor housing 1 is further provided, in its central portion, with a central projection 9, which has a flat end surface 12. The membrane holder 4 has in its central portion, a membrane stretcher hole 10 which is sized to fittingly receive the central projection 9 of the sensor housing 1, and a membrane 3 is held between the periphery of the membrane stretcher hole 10 and that of a ring part opening 5 of the ring-shaped collar 6. The membrane holder 4 and the ring-shaped collar 6 are fastened to each other at a circumferential irregular part 16.
As shown in prior art FIG. 12, the sensor housing 1 is threadedly connected with the ring-shaped collar 6 so as to retain an electrolytic solution in chamber 20 between the membrane 3 and the end surface 12 of the central projection 9. In the conventional gas sensor of the aforementioned structure, however, the electrolytic solution is gradually collected and an excessive quantity of electrolytic solution is retained. The electrolytic solution thus excessively retained is moved when the membrane touches the skin in attachment/detachment of the sensor to/from the human body, whereby the values measured by the gas sensor fluctuate extremely and in most cases do not to return to the original value. Hence, a calibration must be performed repeatedly.
Prior art FIG. 13 shows a jig 17 for threaded connection, which jig is well known as means for solving such a problem. Referring to FIG. 13, the jig 17 is engageable/disengageable with/from the ring-shaped collar 6, and has an elastic member 18 in a part located in the ring part opening 5 of the ring-shaped collar 6. The elastic member 18, being made of rubber or sponge, is adapted to press the membrane 3 when the sensor housing 1 is threadedly connected with the ring-shaped collar 6. The membrane 3 is so pressed in threaded connection that no excessive electrolytic solution is retained between the membrane 3 and the end surface 12 of the central projection 9.
In such a method, however, the jig for threaded connection must be engaged/disengaged with/from the sensor for replacement of the membrane, whereby operation for replacing the membrane is complicated.